This tutorial was originally developed for the 2012 Berlin Hackathon, a face-to-face event. It has been adapted for on-wiki, remote use, as you see it here. As all things wiki, it is a living document and we encourage you to participate in developing and maintaining it.

The content hereafter is focused on established MediaWiki template developers. Everyone is welcome to use it. If you're new to templates on MediaWiki, but have Lua experience, you may want to find out more about templates. Note that some things in Scribunto do not apply to the Lua distribution. If you're a MediaWiki template guru, but new to Lua, you will learn some cool, new magic. If you're new to both, you will find that the MediaWiki community has many people willing to help you learn.

What is different from JavaScript and OO languages we could have used?

Scribunto: how is the Lua implementation in MediaWiki different? Wikitext is not parsed the way you would normally expect. Parser frames are arguments in Scribunto templates. Strip markers replace certain markup, which, if it is passed, will result in placeholders instead of markup. You will need to use the new Namespace:Module.

this tells lua the "scope" of the variable that follows. It can only be accessed right here, locally. The opposite is "global"

p

this is a (local) object (variable) we are creating. We have decided to call it p. We could call it anything we wanted. We will address it as p later, whenever we want to do something with it.

= {}

The equal sign means we're assigning what comes after it to what came before it. The {} means "object", in this and many other script languages like JavaScript. It could have had stuff inside it like {foo = 'bar'}, but we're making it empty and will assign stuff inside it later.

function

Now we're creating a function, which like in calculus is a piece of code that takes values and returns other values. The function will include all code until we get to a keyword "end". This is similar to most popular programming languages.

p.hello

Remember when we made the object "p"? Now we are creating a property inside of that object, called "hello", which is this function. The dot means "property inside" in this and many other script languages.

(frame)

The parens() wrap whatever values we are passing to the function we are creating, the one called "hello". "frame" is the name we give a container of all parameters we'll pass to it. We could call it anything, as long as we consistently refer to it by that name later.

return 'Hello'

"return" says we send this back out of the function we're in, which is called p.Hello, and it will be returned to the object we created, p. "Hello" is the thing we're returning. The single quotes around it mean it's a string, not a variable named Hello that contains some value.

end

Remember when we said that the function would go on until we got to a keyword end? This is that keyword.

return p

Remember the other "return" that sent 'Hello' out of the function called p.Hello? Well THIS return takes whatever happened inside the entire object (which we called p) and sends it outside of that object. In this case, it'll end up being printed to the screen for the user to see or wiki server to parse.

Such a function can access the frame object to get access to the parameters that the template was invoked with. For example, let's change the previous module so that it takes two arguments, like this:

{{#invoke: Sandbox:your username/my_module | hello | hair | brown }}

Our Lua function can use the parameters "hair" and "brown" in either of two ways:

Access them directly as frame.args[1] and frame.args[2], respectively (named arguments can also be used, such as frame.args["title"] or frame.args.title).

Iterate them using the pairs( frame.args ) function, which returns a (name, value) pair for each; this is the best way to code a template that can take a variable number of parameters.

A few pitfalls to avoid:

The parameter values are always strings, even if they are numeric in form; you can use the Lua function tonumber() if you need to treat a value as a number.

However, numbered parameter keys are numbers, even if they are given explicitly in the template invocation (for example, the parameters in both {{#invoke:Sandbox:your username/my_module | hello | world}} and {{#invoke:Sandbox:your username/my_module | hello | 1=world}} are indexed by the number 1, not the string "1"

With explicitly named (or numbered) parameters, leading and trailing whitespace is stripped from name and value. This is similar to template parameters.

An empty string value is not the same as a missing value, which is nil; for example,

{{#invoke:Sandbox:your username/my_module | hello | world }}

results in frame.args[2] being nil (which will cause a script error in the example code hereinbefore), but

{{#invoke:Sandbox:your username/my_module | hello | world | }}

results in frame.args[2] being an empty string (which will not, although the resulting text will look odd). One way to work around this is to assign the desired parameters to a local variable, and replacing nils with empty strings at this step; e.g.,